1. Field of the Invention
The present invention relates generally to methods of routing carrier wavelengths through a fiber optic communications network, and more particularly to a method wherein management data is continuously transmitted by subband quadrature coding along with application data on an optical wavelength carrier signal for routing a carrier wavelength through an optical communications network.
2. Brief Description of the Prior Art
Optical communications systems service an important and expanding portion of current needs. The wide bandwidth provided by lightwave carrier signals allows the transmission of larger quantities of data to be sent in a given time frame than in systems using carriers at lower frequencies. Reliable and economical methods are required for directing/routing the optical signals from a source to a desired destination. FIG. 1 is a simplified diagram of a network 10 for illustrating an example of the routing of signals through a network. Fiber optic cables 12-18 interconnect four nodes or hubs 20-26 providing access to the network 10. A symbol such as item 27 is used to indicate that a line is a fiber optic cable, and will be used for that purpose in the various figures of the drawing. Each node in FIG. 1 includes circuitry for inputting data at 28 and outputting data at 30, the circuitry having a laser source, a controller, and an add/drop module for receiving/dropping and sending/adding a signal from and to the network 10.
The signals transmitted include user application data, and must also contain or be accompanied by a signal with management data identifying the required destination. Each node must have the capability of reading the management data and receiving data that is intended for that node. One prior art method of transmitting management data is illustrated in FIG. 2 wherein a block of management data, noted as a tag 32, is transmitted to identify a required destination. Each node reads the management data, and responds by receiving user data 34 intended for that destination, and causes user data for other destinations to continue around the network 10. A signal is simply transmitted around the ring until it reaches the required destination. Another type of network element is illustrated in FIG. 3 wherein a hub or router 36 evaluates management data, for example from an incoming signal on line 38, and responds by transmitting the signal on the appropriate one of lines 40, 42 or 44.
A disadvantage of the use of a tag 32 or header is that, because it occupies only a finite time frame, it can be lost or misread as a result of system noise. Use of a xe2x80x9cheaderxe2x80x9d is described for example in U.S. Pat. No. 5,438,444 by Tayonaka (column 3, lines 20-43; and column 10, lines 26-34). The technique of using a header to determine a destination is also described in ATM Networks, Principles and Use, by Martin P. Clark, page 13 (a Wiley Teubner publication). Another disadvantage of using a xe2x80x9ctagxe2x80x9d or header to carry management data is that it consumes system bandwidth that could otherwise be used for application data. For example, in ATM cells, a cell length is 53 bytes of which 5 are used for a tag to carry management data, as illustrated in FIG. 2.
Another method that is used to transmit management data is illustrated in FIG. 4. Two laser sources 46 and 48 at different wavelengths are used. Source 1 carries the application data, and source 2 carries the management data. The data input/modulation of the two sources is managed by a controller 50. The two source outputs at 52 and 54 are fed through a multiplexer 56 to place them on a single optic fiber 58. The disadvantage of this method is the cost and complexity of the additional management channel.
It is therefore an object of the present invention to provide an improved method of transmitting network management data through an optical communications network.
It is another object of the present invention to provide a low cost and reliable method of transmitting management data in an optical communications network.
Briefly, a preferred embodiment of the present invention includes a method of transmitting network management data through an optical communications network for routing of discrete wavelengths through the network by controlling switching at the nodes, wherein the management data is impressed on an optical carrier of a particular wavelength through use of a method known as subband quadrature coding (SQC). The optical carrier is modulated at a first frequency, and at a first intensity to impress application data on the carrier. The carrier is additionally modulated at a second frequency, preferably two orders of magnitude less than the first frequency, and at a second intensity less than the first intensity to impress management data on the carrier for routing the carrier through an optical communications network, from a source to a destination. According to the method, the management data is continuously impressed upon the carrier during transmission of the application data. The optical carrier is multiplexed onto the fiber optic communications line, and upon reaching a first node, the carrier is demultiplexed and an intensity sample of the carrier is detected. The management data is then separated out with a low pass filter, whereupon in response the method directs the carrier signal according to the management data.